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What am I missing

Gerald K4NHN on Thu May 08, 2014 2:02 PM User is offlineView users profile

Year: 1984
Make: Isuzu
Model: Pup
Engine Size: 2.2
Refrigerant Type: R-12
Ambient Temp: 88 F
Pressure Low: -20
Pressure High: 120
Country of Origin: United States

My friend brought over his new to him Pup to get it back working. I pulled a vacuum, added a little oil, and charged it up with 30 oz of R-12. All was good and on the way home it quit working. I reclaimed all of the R-12, that I could, pulled a vacuum and then put in 30 oz of R-12 again.. The high pressure goes up to about 125 high and the low went down to about -20 on the low side. I figured that the expansion valve was clogged up and he put a new one in it and we jumped through the same hoops again along with a new drier and o rings. When I pulled a vacuum, watching the low side go down and then the high side slowly went down, not as fast as the low side. I put in about 1 pound before starting the compressor and let it pull in the balance. But the low side kept on going low with the freon shut off. The low side was at about a -15 to -20 and the high side was at about 120... There was very little heat on the high side and the low side was outside temp of about 88 degrees. temp.. Unless we have another bad expansion valve, can the pump be that good on the low side and not make pressure. With these ole Isuzu AC system we do live with a lot of ole parts, about the only thing that we can still get new these days are the expansion valves and driers. I have a couple of evaporators and condensers and a compressor if needed.. I though that the high and low side didn't track very good when pulling the vacuum. After the system was stopped, both gauges didn't equalize. The high stayed high at 120 and the low stayed at -20. So what am I over looking here.

Gerald K4NHN
Cayce, SC

Jag987 on Thu May 08, 2014 4:33 PM User is offline

Stupid question, but any chance that the expansion valve is being installed backwards? If I was working on it, I would start with a good flush to make sure nothing was plugged up. Then add back the correct amount of oil and refrigerant. Your high pressure seems a little low, but it has been a long time since I have done much with R12 so I could easily be wrong. At least the compressor is still working to get that much pressure.
Good luck.

-------------------------
I bought a can of 134a at w**-mart that had a stop leak, oil, and dye in it. It also had a hose and a gauge, so now I'm an AC pro!

Dougflas on Thu May 08, 2014 4:52 PM User is offline

Start looking for your restriction/blockage. If it worked for a while, TXV could not be installed backwards, right?

Jag987 on Thu May 08, 2014 6:07 PM User is offline

Right, it could not be backwards if it worked for a while. But it was replaced so the new one might be.

-------------------------
I bought a can of 134a at w**-mart that had a stop leak, oil, and dye in it. It also had a hose and a gauge, so now I'm an AC pro!

Gerald K4NHN on Thu May 08, 2014 7:44 PM User is offlineView users profile

I didn't install it, but think that it has two different size fittings so that it goes only one way. As soon as I get it back I'll be blowing through the lines, condenser and evaporator checking for blockage. I think that with it able to pull -20 on the low side that the compressor must be in pretty fair shape..

Gerald K4NHN
Cayce, SC

Gerald K4NHN on Sun June 15, 2014 8:53 PM User is offlineView users profile

Well, I got my hands back on this PuP today and I'm still pulling my hair out on it. I recovered the R-12 that was in the system and recharged it with the correct amount of 30 oz by weight. With the outside air at 85 degrees, the low side goes down to a negative 20 and the high side it at 110 which is, I think is about what the static pressure would be for around 85 degrees outside air. I pulled the Freon back out and then pulled the evaporator out to make sure that the expansion valve was put in correctly. With it apart I was able to push air through it with no trouble. I compared it to another under dash unit that I have that came out of an operating system and the both seems to be about the same with air passing through them both... tomorrow I plan to put the other under dash unit in to see it that makes any difference. With the system charged up and after setting for about three hours, the static pressure was still not the same. The high side was at the 110 and the low had moved up to 50 pounds. As I recovered the Freon from the high side the low side stayed at the negative 20 until The high side was almost to several pounds before the low side ever started to move. The drier that was used is this one. A/C Receiver Drier UAC RD 3167C. It wasn't marked either direction, in or out, I connected the vacuum pump to the connect and pulled a vacuum from that port with both hard line connection open and then put my finger on both and it was pulling a vacuum on both ports. There wasn't any marking on it for in or out but it doesn't seem to be plugged up and no blockage.. can I assume that the compressor is OK with it pulling a neg 20 on the low side. The high side is low, but I sort of wrote that off as the compressor isn't getting any Freon to build up pressure. Am I correct in assuming this.

Gerald K4NHN
Cayce, SC

mk378 on Sun June 15, 2014 10:26 PM User is offline

Correct, with a total blockage the high side will stay low because all the refrigerant condenses in the condenser and receiver at ambient temperature and just sits there. If you're measuring high side before the condenser, it could be the condenser that's blocked. Check every part that is in between the high and low service ports. If the receiver drier has a sight glass on it, the port that is connected to the sight glass will be the outlet. Note that some units appear not to have a sight glass but there actually is one, it is covered over with masking tape that wasn't removed after the paint job.

Edited: Sun June 15, 2014 at 10:28 PM by mk378

Gerald K4NHN on Sun June 15, 2014 11:58 PM User is offlineView users profile

I'm measuring the low side right at the compressor and the high side is measured on the drier, just after the condenser. yea, I have the sight glass and knew about the little dot covering it up. I did check the out port today on the drier and I can see my small wire I used to push into it, in the sight glass. So that's installed correctly. I'll see what replacing the evaporator and expansion valve tomorrow does for me.

Thanks for the help.

Gerald K4NHN
Cayce, SC

Gerald K4NHN on Tue June 17, 2014 9:38 PM User is offlineView users profile

mk378
Well I'm back. After a couple of days working on the PUP, I think that I have it going real well. I'd work on it in the cool of the mornings and then late afternoon. It's been so hot here and the humidity has been high with it feeling like it 104 and 105 degrees.

I recovered the Freon and charged it twice. I took the evaporator in and out twice also. I pulled the expansion valve and couldn't see anything wrong and the evaporator seemed to not have any blockage, so after doing all of this twice, I decided to put the used unit in that I've had on my shelf for my back up when I might need one. But this guy is a friend and he needs air in this weather we're here in the South. So finally when I charged the little PUP up everything fell into place and now it has 40 degree air coming out of the center vent even after idle in 85 degree heat. These Pup's don't have enough air coming across the condenser at idle and they suffer because of that. I needed to add a little extra air from a fan out front of the truck and the pressures looked real good. I measured the temp drop across the condenser and I think it was about 35 to 40 degrees. My short term is showing up.. The expansion valve was new so I'm guessing that it was OK and it was the evaporator that was plugging up.

Just want to say thanks for the help that all of you gave me and the support. Now I need to check my PUP, it doesn't cool as well as the one that I just worked on. I've been thinking that my compressor may be getting weak..

Gerald K4NHN
Cayce, SC

mk378 on Wed June 18, 2014 12:09 AM User is offline

My guess would be a bad TXV. New parts aren't necessarily good. If the gas has leaked out of the sensing bulb and diaphragm side, it is going to close prematurely. This could also happen if the bulb or tube was damaged during installation.

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